The present disclosure generally relates to a method and apparatus for preparing gypsum products from starting materials including calcined gypsum and water, and more particularly relates to an improved apparatus for use in conjunction with a slurry mixer used in supplying agitated gypsum slurry to a wallboard production line.
It is well known to produce gypsum products by dispersing calcined gypsum in water to form a slurry, then casting the slurry into a desired shaped mold or onto a surface, and allowing the slurry to set to form hardened gypsum by reaction of the calcined gypsum (calcium sulfate hemihydrite or anhydrite) with the water to form hydrated gypsum (calcium sulfate dihydrate). It is also well known to produce a lightweight gypsum product by mixing an aqueous foam into the slurry to produce air bubbles. This will result in a desired distribution of voids in the set gypsum product if the bubbles do not escape from the slurry before the hardened gypsum forms. The voids lower the density of the final product, which is often referred to as “foamed gypsum.”
Prior apparatus and methods for addressing some of the operational problems associated with the production of foamed gypsum are disclosed in commonly-assigned U.S. Pat. Nos. 5,638,635; 5,643,510; 6,494,609; and 6,874,930; all of which are incorporated by reference. The present invention relates generally to mixers used in the formulation of gypsum slurries in the production of gypsum wallboard.
A gypsum wallboard mixer typically includes a housing defining a mixing chamber with inlets for receiving sources of calcined gypsum and water, among other additives well known in the art. The mixer includes an impeller or other type of agitator for agitating the contents to be mixed into a mixture or slurry. Such mixers typically have a rectangular discharge gate or slot with a cutoff block or door. The discharge gate controls the flow of slurry from the mixer, but is difficult to adjust to change slurry flow when product requirements change, such as when thicker or thinner wallboard is desired.
Foam and/or other additives are typically added through a foam injection port on an outer side wall of the discharge gate through which aqueous foam or other desired additives, such as retarders, accelerators, dispersants, starch, binders, and strength-enhancing products including poly-phosphates, sodium trimetaphosphate, and the like, after the slurry has been substantially mixed. To promote more uniform mixing of foam or other additives into the gypsum slurry, designers have the goal of preventing the foam and/or additives from flowing backwards and entering into the mixing chamber to prematurely mix with the gypsum slurry.
An inlet opening of the discharge gate for receiving the mixed slurry is typically equipped with lump bars or grating for preventing slurry lumps from entering into the discharge gate. As a result, in some applications, the inlet opening is configured to be large and oversized, and causes slurry flow problems when the foam and/or additives are injected into a cavity of the discharge gate. Specifically, the large inlet opening of the discharge gate makes it difficult to match the cavity area to the volume of mixed slurry flowing through from the inlet opening to an outlet opening of the discharge gate. If the grate is not full, lumps can form from eddy patterns created by the slurry flow in the mixer.
Thus, several factors combine to provide a gypsum wallboard mixer that operates properly, and these include the size of the discharge gate, whether or not lump bars obscure the gate opening, the volume of slurry in the mixer, and the point of introduction of foam into the slurry.
Therefore, there is a need for an improved discharge gate having the injection port that provides the desired 90° injection angle, and the cavity area that matches the volume of mixed slurry flowing through the mixer.